1. Technical Field of the Invention
The present invention relates to the administration of agents mimicking DOPAchrome tautomerase activity, for protecting the melanocytes of the hair follicle. In particular, the agents mimicking DOPAchrome tautomerase activity combat the disappearance of the melanocytes of the hair follicle by maintaining and/or regenerating the population of active melanocytes of the bulb and of quiescent melanocytes of the upper or top region of the hair follicle.
2. Description of Background and/or Related and/or Prior Art
The hair follicle is a tubular invagination of the epidermis which extends up to the deep layers of the dermis. The bottom part, or hair bulb, itself comprises an invagination in which is the dermal papilla. The bottom part of the bulb is a zone of cellular proliferation where the precursors of the keratinized cells constituting the hair are found. The ascending cells derived from these precursors are gradually keratinized in the top part of the bulb, and this group of keratinized cells will form the hair shaft.
The color of head hair and of body hair depends in particular on the presence in variable quantities and ratios of two groups of melanins: eumelanins (brown and black pigments) and pheomelanins (red and yellow pigments). The pigmentation of head hair and of body hair requires the presence of melanocytes in the bulb of the hair follicle. These melanocytes are in an active state, that is to say that they synthesize melanins. These pigments are transmitted to the keratinocytes intended to form the hair shaft, which will result in the growth of a pigmented head hair or body hair. This structure is called hereinafter “the follicular unit of pigmentation”.
In mammals, melanogenesis involves at least three enzymes: tyrosinase, DOPAchrome tautomerase (TRP-2, for Tyrosinase Related Protein 2) and DHICAoxidase (TRP-1, for Tyrosinase Related Protein 1).
Tyrosinase is the enzyme which initiates the biosynthesis of melanins. It is also described as being the enzyme which limits melanogenesis.
TRP-2 catalyzes the tautomerization of DOPAchrome 5,6-dihydroxyindole-2-carboxylic acid (DHICA). In the absence of TRP-2, DOPAchrome undergoes spontaneous decarboxylation to form 5,6-dihydroxyindole (DHI).
DHICA and DHI are both precursors of pigments, TRP-1 oxidizes DHICA molecules to form quinone derivatives (Pawelek J M and Chakraborty A K. The enzymology of melanogenesis. In: Nordlund J J, Boissy R E, Hearing V J, King R A, Ortonne J-P. The Pigmentary System: Physiology and Pathophysiology, New York: Oxford University Press; 1998. p. 391-400).
The three enzymes, tyrosinase, TRP-2 and TRP-1, appear to be specifically involved in melanogenesis. Furthermore, the activity of these three enzymes has been described as necessary for the maximum activity of biosynthesis of eumelanins.
The expression of TRP-2 has been observed in the hair of black mice, both in the active melanocytes of the bulb and in the quiescent melanocytes of the outer epithelial sheath. Furthermore, it is known that the DOPAchrome tautomerase activity is increased during the anagen phase in black mice. However, no clear correlation has been established between the expression of TRP-2 and the intensity of the pigmentation (Sturm et al., 1995).
Moreover, TRP-2 has also been described as conferring on the melanocytes expressing it resistance to DNA damaging agents such as cis-diaminedichloroplatinum(II) (Chu et al., 2000 and Pak et al., 2000). These results suggest that TRP-2 might also be involved in a function independent of melanogenesis; the enzyme could play a cytoprotective role.
Head hair and body hair undergo a cycle. This cycle comprises a growth phase (anagen phase), a degenerative phase (catagen phase) and a resting phase (telogen phase) after which a new anagen phase will develop. Because of this hair cycle, and unlike the epidermal pigmentation unit, the follicular pigmentation unit must also be cyclically renewed.
This process was recently described in humans (Commo S. and Bernard B., 2000, Pigment Cell Res., 13:253-259). It has more particularly been shown that during the telogen-anagen transition, a portion of the inactive melanocytes contained in the telogen capsule proliferate, become positioned around the dermal papilla of the nascent bulb and start to express enzymes necessary for the synthesis of melanins: this population of melanocytes corresponds to the active melanocytes of the bulb. In parallel, the other portion of the melanocytes remains inactive in the top region of the hair follicle: this population of melanocytes corresponds to the quiescent melanocytes of the top region of the hair follicle.
These melanogenic enzymes will be expressed in the melanocytes of the bulb during the entire duration of the anagen phase but will no longer be expressed during the catagen and telogen phases. The normal cycle for the melanocytes in the human hair follicle requires the presence of quiescent melanocytes in the top region of the hair follicle, a region otherwise called “reservoir”, which will be cyclically activated in order to regenerate the follicular pigmentation unit. This mechanism of cell renewal which participates in maintaining pigmentation is specific to the follicular pigmentation unit; it is not found in the epidermal pigmentation unit.
It is accepted that canities (natural whitening or graying of the hair) is associated with a decrease in melanin in the hair shaft. The cause of this decrease has not been elucidated to date. Several hypotheses have been advanced; it could be linked to a decrease in the melanogenic activity, by analogy with the mechanism of pigmentation of the skin, but also to an impairment in the transfer of melanins or a decrease in the number of melanocytes in the bulb (Tobin and Paus, 2001); and no demonstration in hair pigmentation has to date made it possible to validate either of these hypotheses.
Applicants have now demonstrated two results which validate for the first time the hypothesis according to which canities could be linked to a decrease in the number of active melanocytes in the bulb and a decrease in the number of quiescent melanocytes in the top region of the hair follicle. This premature decrease and/or disappearance of the melanocytes is specific to the hair follicle and does not visibly affect the epidermis.
To date, it was indeed considered that quiescent melanocytes were present in the hair follicles of white hair (Takada et al., 1992, Horikawa et al., 1996, Jenner and Randall 2000).
Also, Applicants have now observed that the progression of canities is associated with a decrease in the number of melanocytes in the hair bulbs which, although in a limited number, synthesize and transfer melanins. Applicants have also observed, unexpectedly and surprisingly, that the population of quiescent melanocytes in the top or upper region of the human hair follicle (also called “reservoir”) is also reduced during the canities process, white hair now possessing only a few—or even no—melanocytes, unlike the infundibulum and the epidermis near this white hair. This disappearance affects prematurely and specifically the melanocytes contained in the hair.
It therefore appears necessary to combat the disappearance of the melanocytes of the human hair follicles, a process which affects both the active melanocytes of the bulbs and the quiescent melanocytes of the top region of the hair follicles, in order to combat canities.
Applicants have also observed, unexpectedly, that the enzyme TRP-2 is not expressed in the melanocytes of pigmented (brown, black and red) human hair follicles in Caucasian, Asian and African individuals. This enzyme is not detected either in the active melanocytes of the bulb, or in the quiescent melanocytes of the top region of the human hair follicle whereas it is expressed in the epidermis and the infundibulum of Caucasian, African and Asian individuals. The absence of TRP-2 is associated with the premature disappearance of the melanocytes which do not express it, that is to say the quiescent melanocytes of the top region of the hair follicle and the active melanocytes of the bulb.